Preheat oven to 200°C. Grease a 25 x 16cm slice tin with butter. Peel and thinly slice potatoes and celeriac. Arrange ½ the potato and ½ the celeriac in prepared tin, overlapping slightly. Sprinkle over ½ the sage leaves and

½ the cheese.

Pour over ½ the cream and season with salt and pepper. Arrange remaining potato and celeriac in tin, overlapping slightly. Pour over remaining cream. Season with salt and pepper. Sprinkle over remaining cheese. Bake for 45-50 minutes or until golden and potato is soft and cooked through.

Heat oil in a small frying pan over a high heat until hot. Add remaining sage, in batches, and cook for 30 seconds. Drain on kitchen paper. Sprinkle over bake. Slice bake to serve.

For extra flavour, add mustard or dried Italian herbs to the cream, before pouring over the potatoes.